Some electrophotographic image forming apparatuses, for example, electrophotographic printers, electrophotographic copy machines, and the like are equipped with a belt driving unit which circularly moves an endless belt while suspending the belt with multiple belt suspending rollers. A belt driving unit such as the one described above is problematic in that as the belt is circularly driven, it tends to shift in the widthwise direction of the belt (direction roughly perpendicular to moving direction of belt).
There are various methods for controlling this lateral movement of the belt. One of them is disclosed in Japanese Laid-open Patent Application H05-26314. According to this patent application, the belt driving unit is provided with multiple belt suspending rollers, and one of the multiple belt suspending rollers is utilized as a belt position adjustment roller, that is, a roller for correcting the belt in terms of lateral shift. Further, the belt driving unit is provided with a belt position detection roller which is independently rotatable from the roller for correcting the belt in terms of lateral shift. The belt position detection roller is positioned next to one of the lengthwise ends of the belt suspending roller in order to correct the belt in terms of lateral shift. Further, the belt driving unit is provided with a string with a preset length. One end of the string is attached to the peripheral surface of the belt position detection roller. As the belt laterally shifts, it comes into contact with the belt position detection roller. Thus, the friction between the belt and belt position detection roller causes the belt position detection roller to rotate. Thus, the string is wound around the peripheral surface of the belt position detection roller, causing the rotational axis of the belt position adjustment roller to tilt in such a manner that the belt is made to shift toward the other end of the belt position adjustment roller.
However, the structural arrangement disclosed in Japanese Laid-open Patent Application H05-26314 makes it necessary for the other end (opposite end from belt position detection roller) of the belt position adjustment roller to be supported. Thus, the distance by which the belt position adjustment roller has to move to cancel the unwanted lateral shift of the belt is substantial. In other words, it is slow in response. Thus, it needs to be improved in responsiveness, and also, requires a large space in order to allow the belt position adjustment roller to sufficiently tilt for the belt to be put back into its proper position.